Bacidia gigantensis, a green to greenish-brown, sorediate, crustose to microsquamulose lichen is described as new to science. It is corticolous on mature Thuja occidentalis trees throughout Sleeping Giant Provincial Park on the Sibley Peninsula along the north shore of Lake Superior, Ontario. The species is unusual within Bacidia due to the production of asexual propagules and homosekikaic acid. Placement in Bacidia is based on the characters of the sexual reproductive structures (biatorine apothecia with a true exciple, Bacidia-type asci, and sigmoid, acicular, ascospores), ITS sequence data and molecular phylogenetic analyses of mtSSU sequence data. The latter also inferred a relationship to Bacidiopsora, a tropical genus that has been proposed for synonymy with Bacidia and includes other species with similar chemistries.